Radio frequency ablation (RFA) involves the destruction of undesirable cells by generating heat through agitation caused by the application of alternating electrical current (radio frequency energy) through tissue. Various RFA ablation devices have been designed to perform this treatment. For example, U.S. Pat. No. 5,855,576 describes an ablation apparatus that includes an array of wire electrodes that are deployed through a catheter. Proximal ends of the wires are connected to a RF power source (generator), with the distal ends projecting generally radially and uniformly spaced apart into a target tissue structure (e.g., a tumor) from the catheter distal end. The wire ends act as electrodes that may be energized in a monopolar or bipolar fashion to heat and necrose tissue within a defined volumetric region of target tissue. The current can flow between closely spaced energized wire electrodes or between an energized wire electrode and a larger, common electrode located remotely from the tissue to be heated. In order to assure that the target tissue is adequately treated and limit damage to adjacent healthy tissues, it is desirable that the array formed by the wires within the tissue be precisely and uniformly defined. In particular, it is generally desirable that the independent wires be evenly and symmetrically spaced-apart so that heat is generated uniformly within the desired target tissue volume. The ablation device may be used either in an open surgical setting, in laparoscopic (small incision) procedures, or in percutaneous (through the skin) interventions.
For example, FIGS. 1A to 1D show how a thermal lesion is created using the above described ablation apparatus. Using conventional imaging methods such as ultrasound, an array 2 of wires 4 is positioned strategically within the targeted area of tissue and energized with electrical current. Initially, a thermal lesion 6 begins to form at the tips of the wires 4 (FIG. 1A). The lesion 6 expands along the wires 4 back toward the center of the array 2, indicated by directional arrow 7 (FIG. 1B), then outward and between the wires 4, indicated by directional arrow 8 (FIG. 1C), until the full lesion 6 is formed (FIG. 1D).
Due to physical changes within the tissue during the ablation process, the desired thermal lesion 6 illustrated in FIG. 1D is typically difficult to achieve in a single RF application. For example, the concentration of heat adjacent the wires 4 often causes the local tissue to desiccate, thereby reducing its electrical conductivity. Also, the tissue temperature proximate the wires 4 may approach 100° C., so that water within the tissue vaporizes. As this desiccation and/or vaporization process continues, the impedance of the local tissue may rise to the point where current can no longer pass into the surrounding tissue. As such, depending on the rate of heating and how far the wire electrodes are spaced from each other, ablation devices that have multiple spreading wires may fail to create complete and uniform lesions. While wire electrodes can be repositioned to treat additional tissue, the precise movement required for this task is difficult to accomplish. Furthermore, the nature of percutaneous treatment inherently limits the ability to precisely place medical devices and sensors using traditional visual methods.